Can it be a way to say "you have 4 sons", with "you count" used figuravely, as it would be possible in French?
Yes, but it's rare. Ovid: "donec eris felix, multos numerabis amicos", "you'll have lots of friends, as long as you're prosperous".
Why numeras, not numeris?
Numero, numerare is a first conjugation verb, thus the second person singular indicative active is numeras.
Numeris can either be the dative or ablative plural of the noun numerus, numeri (number).
See Moopish answer. Or maybe you were looking for the second peron plural conjugation, i.e. it should be nummeratis.
The comments talk about filios. I came over because it now translates sons into filias. Isn't that wrong?
My audio is saying "filaos". And it's even accepting that as a typed answer (though generally the mis-spelling flags seem to be missing).
I hear "filaos" too. Confused me, but I spelled it "filios "